Bushing structure for electric apparatus



Nov. 21, 1961 J. A. OPPIEL BUSHING STRUCTURE FOR ELECTRIC APPARATUS 2Sheets-Sheet 1 Filed Sept. 30, 1959 l ZW/M:

Inventor: John A. Oppel,

His Attovneg.

N 1, 1961 J. A. OPPEL 3,009,983

BUSHING STRUCTURE FOR ELECTRIC APPARATUS Filed Sept. 30, 1959 2Sheets-Sheet 2 Inventor: Altai-H1 A. Oppel,

" His Attorney.

Patented Nov. 21, 1961 3,009,983 BUSHING STRUCTURE FOR ELECTRICAPPARATUS John A. Oppel, Aldan, Pa., assignor to General ElectricCompany, a corporation of New York Filed Sept. 30, 1959, Ser. No.843,589 5 Claims. (Cl. 174--18) This invention relates to a high voltageterminal bushing for electrical apparatus and, more particularly, to aterminal bushing of the type that is filled with a pressurizedinsulating gas.

The usual high voltage bushing comprises a central conductive studsurrounded by housing means formed from a pair of tubular insulatingshells. The insulating shells are customarily maintained in compressionby clamping devices mounted at opposite ends of the central stud andbearing against opposite ends of the housing means. To allow the stud toexpand and contract in response to temperature changes without damagingthe in sulating shells, it is customary to provide spring means betweenone end of the housing and the adjacent clamping device. The spacebetween the stud and the housing constitutes a chamber filled with adielectric medium. Suitable gaskets are provided to seal this chamberfrom the surrounding atmosphere, and the spring means holds thesegaskets in compression in order to maintain the chamber sealed despiteunequal expansion and contraction of the bushing parts. i

In a bushing of the gas-filled type, pressurized gas fills the chamberbetween the stud and the housing means. This pressurized gas tends toproduce forces opposing the action of the spring means, and this hasresulted in complications. One of these complications is that it hasbeen necessary heretofore to use relatively large and unwieldy springmeans to withstand the opposing forces resulting from the pressurizedgas and, at the same time, to perform its intended function ofmaintaining the insulating shells and the gaskets suflicientlycompressed. The fact that the'spring means has been large and unwieldyhas necessitated locating it outside the bushing housing, thus undulyincreasing the overall length of the bushing.

An object of my'invention is to construct a gas-filled bushing in such amanner that a relatively small and weak spring device located internallyofthe bushing housing may be relied upon for compressing the gaskets andthe insulating shells to the required extent despite the presence of ahighly pressurized gas within the bushing.

Another object is to utilize the pressurized gas within the bushing forproviding a portion of the force for maintaining one of the insulatingshells and certain of the gaskets in compression.

In a preferred embodiment of my invention, 1 utilize my bushing forcarrying electric power to .andfrom a location within a tank containingpressurized insulating gas. a t

In such embodiment, another object of my invention is to utilize thepressure of the gas Within the tank for helping to maintain one of theinsulating shells in compression without allowing the pressurized gaswithin the tank to reduce the effectiveness of the spring means inmaintain- Another object is to provide mounting means for. the bushingwhich permits the bushing to be quickly installed and removed as aself-contained unit and which is capable of withstanding high forcestending to force thebushing out of its mounting, all Without relianceupon the heavy ing the other insulating shell and the gasketscompressed.

an opening in the tank. The bushing comprises a conductive stud andhousing means including a pair of insulating shells surrounding theconductive stud to insulate the stud from the tank. The housing meansforms a chamber about the stud which contains pressurized dielectric gasat a pressure lower than that of the gas within the tank. Adjustablymounted on the inner end of the stud is stop means rigidly bearingagainst the inner end of the housing means. Bearing against the outerend of the housing means is an annular sealing plate that closes off theouter end of the chamber. Secured to the outer end of the stud is apiston-like member of larger external diameter than the stud. Thispiston-like member is arranged for telescopic movement within thesealing plate during contraction and expansion of the stud and isprovided with a seal disposed between the piston-like member and thesealing plate. Spring means acting between the piston-like member andthe sealing plate is provided for loading the insulating shells incompression and for holding the sealing plate in sealed relationshiprelative to the housing means. The pressurized gas within the insulatingchamber of the bushing acts through the piston-like memher to applytension forces to the stud through a path effectively bypassing thespring means. The spring means is located internally of the housingmeans at the outer end of the bushing. I

For a better understanding of my invention, reference may be had to thefollowing description taken in conjunction with the accompanyingdrawings, wherein:

FIG. 1 is a side elevation view partly in section of an air blastcircuit breaker comprising two bushings, each embodying one form of myinvention.

FIG. 2 is a sectional View of one of the bushings disclosed in FIG. 1.

Referring now to FIG. 1, there is shown a gas-blast I circuit breakercomprising a metallic tank 10 filled with a pressurizedarc-extinguishing gas, such as air. Disposed within the tank 10 are twopairs 11 and 12 of separable contacts connected in series-circuitrelationship. Each pair of separable contacts comprises a stationarycontact 13 or 14 and a movable contact 15 or 16. The movable contacts 15and 16 are adapted to be simultaneously separated from their respectivestationary contacts 13 and 14 by means of an operating mechanismgenerally indicated at 18 acting through a set of connecting links 19.

The operatingmechanism and the contacts form no part of the presentinvention and therefore have not been shown in detail in FIG. 1.Preferably, however, these parts are constructed as shown and claimed inapplication S.N. 642,100, Beatty, filed February 25, 1957, now PatentNo. 2,911,492, and assigned to the assignee of the present invention.

When the contacts are separated by the operating mechanism 18, an arc isformed across each inter-contact gap. This arc is quickly extinguishedby the flow of pressurized gas from the tank 10 to the exterior of thetank via a path extending through the inter-contact gap and through anozzle structure 21, as is indicated by the arrows 20. This flow ofpressurized gas is controlled by suitable valve means (not shown)internally of the nozzle structure 21, and such valve means interruptsthe flow of pressurized gas as soon as the arc is extinguished.

The stationary contacts 13 and 14 are respectively supported on theinner ends of terminal bushings 25 and 26 extending through alignedopenings provided in the wall or" tank 10. These bushings each havecentrally disposed conductive studs 27 and 28 for carrying electricpower to ings 25 and 26 serve to electrically insulate their respectiveconductive studs from the tank 10, as will soon be described. Since thebushings 25 and 26 are substantially identical, only one will bedescribed in detail.

Referring now to FIG. 2, which is a detailed crosssectional view ofbushing 25, it can be seen that the bushing comprises a tubular housing30, 31, 32 surrounding the conductive stud 27. This tubular housing iscomposed of a pair of tubular insulating shells 3t) and 31, preferablyof porcelain, and a metallic tubular support 32 disposed between the twoshells 30 and 31. The inner porcelain shell 30 is disposed between theinner end of. the stud 27 and the tank 16, whereas the outer porcelainshell 30 is disposed between the outer end of the stud 27 and the tank10. Sealing plates 34 and 36 are disposed at opposite ends of thehousing 30-32, and through these sealing plates are transmitted forcestending to compress the insulating shells 39 and 31, as will soon beapparent. A portion of the total force acting through the end plates 34and 36 to compress the insulating shells 30 and 31 is derived from aplurality of compression springs 37 disposed Within the outer insulatingshell 31. Each of these compression springs 37 bears at one end againsta suitable nut 38 attached to a bolt threaded into the end plate 36 andat its other end against a forcedistributing washer 40 surrounding thestud 27 and abutting against a piston-like members 42 that is rigidlyattahced to the stud 27. it will therefore be apparent that the springs37 tend to force the end plate 36 to the left and thepiston-like member42 to the right in FIG. 2.

The piston-like member 42 is prevented from moving to the right by astop 44 which is threaded on the other end of the stud 27 and whichcarries jack screws 46 bearing the outer face of the lefthand sealingplate 34. Thus, the springs 37 urge the sealing plates 34 and 36 againstthe insulating shells 30 and 31 in a direction to compress the shells.The force exerted by thme springs 37 can be adjusted simply by adjustingthe jack screws 46, which are threaded in the stop 44. When the jackscrews 46 are adjusted in a direction to increase the spacing betweenthe parts 44 and 34, they force the piston 42 to the left to furthercompress the springs 37 thereby increasing the insulator compressingforces. correspondingly, adjustment of the jack screws 46 in an oppositedirection decreases the forces exerted by springs 37. Even though thesprings 37 are located internally of the bushing housing 30-32, it is asimple matter to obtain an indication of the extent to which they arecompressed. This can be done simply by measuring the distance between ashoulder 48 provided on the piston 42 and the other surface of sealingplate 36.

Between the housing 30-32 and the stud 27 there is a closed,generally-cylindrical chamber 50 sealed at its opposite ends by thesealing plates 34 and 36. This chamber 50 contains a pressurized gas ofhigh dielectric strength, such as sulfur hexafluoride. This pressurizedgas provides the dielectric strength necessary to prevent breakdownsbetween the stud 27 and the tubular metallic supporting ring 32, whichis at the same potential as the tank 10. For preventing leakage of thepressurized gas from chamber 50 or from the tank into the chamber 50,suitable gaskets 52 are provided at opposite ends of each of theporcelain shells 30 and 31. These gaskets 52 are compressed by thepreviously-described insulator compressing forces transmitted fromsprings 37 through the sealing plates 34 and 36, and, thus, the gasketsare effective to preclude leakage at both ends of each of the twoinsulating shells 30 and 31.

Leakage of pressurized gas from the chamber 50 through the sliding jointbetween the outer sealing plate 36 and the piston 42 is prevented bymeans of a suitable O-ring 53 located in an annular groove in the innerperiphery of the sealing plate 36. A similar O-ring 54 is provided inthe inner periphery of the sealing plate 34 to prevent leakage ofpressurized gas from the chamber 50 through the sliding joint betweenthe sealing ring 34 and the conductive stud 27. Adjacent the O-ring 54is another O-ring 55 which prevents leakage of pressurized air from thetank 10 along the outer periphery of the conductive stud 27. The regionbetween the O-rings 54 and 55 is at atmospheric pressure by virtue of apassage 56 extending through the tubular stud 27 into communication withthe bore of the tubular stud 27. The bore of the tubular stud 27 is atatmospheric pressure by virtue of a passageway 58 extending through thepiston member 42 to the surrounding atmosphere and by virtue of a plug59 at the inner end of the stud 27 preventing communication between thebore 27 and the interior of tank 10. This double O-ring arrangement 54,55 with the intervening space vented to atmosphere is advantageous inpreventing any interchange of gases within the bushing 25 and the tank10 inasmuch as any leakage past either O-ring is vented to atmosphererather than being allowed to contaminate the other gas.

The pressurized gas that is contained within the chamber 50 acts on thesealing plates 34 and 36 in a direction to oppose the action of thesprings 37 in compressing the gaskets 52 and the insulators 30, 31. Towithstand these opposing forces and at the same time provide therequired force for compressing the gaskets and insulators, it has beennecessary heretofore to use considerably heavier springs than thoseshown. In the disclosed bushing, springs of reduced size can be utilizedfor performing these functions because a portion of the forces resultingfrom the pressurized gas in chamber 50 is borne by the piston 42, thussubtracting from the forces that would have been exerted by the gas onthe sealing plate 36 had the piston 42 been absent. Not only does thepresence of piston 42 reduce the forces exerted on sealing plate 36, butalso any forces exerted on piston 42 are transmitted through the stud 27to inner insulator 30 in the form of compressive loading on theinsulator 30 and the gasket 52 at the opposite ends of the insulator 30.The magnitude of the forces so transmitted is dependent upon thedifference between the area enclosed by the 0- ring 53 and that enclosedby the O-ring 54. To the ex tent that forces are transmitted in thismanner through the piston 42 and the stud 27 to force the sealing plate34 against the insulator 30, the springs are additionally relieved ofthe burden of withstanding the tendency of the pressurized gas inchamber 50 to separate the end plates 34 and 36 from the housing 30-32.

By enabling springs of reduced size to be relied upon for performing therequired compressing functions, it has been possible to locate thesprings inside the housing 30-32 instead of outside the housing as hasgenerally been the practice heretofore. By locating the springs insidethe housing 30-32, substantial reductions in the overall length of thebushing have been achieved as compared to bushings where the springshave been externally located. To prevent electrical stressconcentrations from occurring near the sharp edges of the springassembly, a shield 60 of smooth external configuration is provided forenclosing the springs 37. Because the internal region at the outboardend of the bushing is a region of low dielectric stress in comparison tothose regions closer to the metallic supporting ring 32 where the gap issmaller, no significant reductions in breakdown voltage result fromlocating the enclosed spring assembly internally of the bushing housing.

Another advantage derived from locating the springs at the outboard endof the bushing (instead of at the inboard end) is that the position of,the contact assembly 13 remains virtually unchanged despite changes inthe pressure of the gas within the tank 10 or the bushing 25, anddespite thermal expansion of the conductor 27. Changes in gas pressuredo not significantly change the contact position because the position ofthe contact is determined by the rigid parts, 30, 34, 44, 46 which aresubstantially unafiected in dimension by variations in pressure withinthe tank 10. Thermal expansion of coirductor27 does not significantlychange the contact position because conductor 27 is free to expand atits outer end without displacing its inner or contact end.

It is to be noted that the springs 27 maintain the gaskets 52 and theinsulators 31 and 32 compressed despite unequal expansion of theconductor 27 and the housing means 30-32. The sliding relationship ofthe piston 42 within the sealing plate 36 allows this independentexpansion and contraction to occur without damage to the insulators.

It has been proposed in certain electrical apparatus containingpressurized gas to allow for free communication between the insulatingspace within the bushing and that within the remainder of the apparatus.In contrast to this approach, I isolate the insulating chamber 50 withinthebushing from the space Within tank 10. As a result of this isolation,I am able to utilize for the bushing a gas which is more suited forinsulating purposes and for the tank a gas which is more suited in thisparticular device for arc-extinguishing purposes. As a further result ofthis isolation, I am also free to use Widely different pressures in thetank 10 and in the bushing chamber 50, selecting for each of thesespaces the pressure best suited for the intended function. In thedisclosed breaker, the pressure within the tank 10 is between 400 and500 psi. whereas the pressure Within the chamber 50 of the bushing isonly about 40 or 50 psi.

By using this materially lower pressure in the bushing, the size of mysprings and the Wall thickness of my insulator 31 can be drasticallyreduced as compared tothe spring size and the wall thickness that wouldbe needed if the high pressure tank space were allowed to freelycommunicate with the chamber 50.

The higher pressure within tank 10 helps to maintain the inner insulator30 in compression and yet does not, in any significant way, reduce theeffectiveness of the spring means 37, 38 in performing its desiredfunction inasmuch as the higher pressure gas is isolated from theinsulating chamber 50 of the bushing.

For holding the bushing 25 in place within its opening in the Wall oftank 10, an annular locking ring 70 having a radially-extending split isprovided. This locking ring 70, which has an unstressed diameterslightly smaller than the diameter of the groove 72 in which it isfitted, is

snapped into this external groove 72 in the outer periphery of thetubular supporting member 32 and bears against the internal face of areinforcing ring 73 welded to the tank wall 10. The tubular supportingmember 32 has a flange external to the tank 10, and this flange carriesjack screws 75 threaded thereinto. When these jack screws 75 aretightened, the reinforcing member 73 of the tank wall is clamped betweenthe split locking ring 70 and the jack screws 75. The high gas pressurewithin tank 10 acts in a direction to force the bushing 25 out of thetank, but the split ring 70 easily withstands this relatively high forceinasmuch as the resultant stresses are distributed generally uniformlyabout substantially the entire periphery of the support member 32. Asuitable O-n'ng seal 76 is provided about the outer periphery of thesupport member 32 to prevent leakage of pressurized gas therearound.

The split-ring type fastening means 70, 72, 75 is highly advantageousfor a number of diiferent reasons. First of all, it allows the bushing25 to be quickly and easily installed in the breaker or removed from thebreaker.

Installation of the bushing is accomplished simply by slipping thebushing minus the split ring 70 into the tank opening from outside thetank, then snapping the split ring 70 in place from inside the tank, andthen tightening up on the jack screws 75. Removal of the bushing isaccomplished by first loosening the jack screws slightly, removing thesplit locking ring 70, and then sliding the bushing out. It will benoted that the bushing is installed and removed as a completelyassembled unit. This not only contributes to the ease and speed at whichthese steps can be performed but also allows the bushing to be testedapart from the breaker and allows it to be assembled at a moreconvenient location. The split ring type fastening means 70, 72, 75 ismuch less expensive than the bolted connections previously used and alsocan be sealed by a simple O-ring (as shown at 76) without the need forproviding seals about each bolt hole as has frequently been the casewhen conventional bolted COD: nections were used. It is to be noted alsothat the bolts needed for a conventional bolted connection would have tobe quite large in view of the high pressure in the tank 10 and therelatively large area of the tank opening. Split ring 70 can easilywithstand this load since the load is distributed substantiallyuniformly about the entire outer periphery of the support member 32.

It should be noted that internally of the porcelain shell 31, there isprovided a tubular sleeve 80 of insulating material having an outsidediameter slightly less than the inside diameter of the porcelain shell31. The purpose of this sleeve 80 is to minimize the volume of gas thatis available to eject porcelain particles in the'remote event that theporcelain shell 31 should become severely damaged. In this regard, onlythe small volume of gas between the sleeve 80 and the shell 31 isavailable for such ejection. The sleeve 80, which has a burstingstrength exceeding that of the porcelain shell 31, would effectivelyisolate the shell 31 from the gas inside of the sleeve 80 under theseconditions.

While I have shown and described a particular embodiment of myinvention, it will be obvious to those skilled in the art that variouschanges and modifications by Letters tank and its outer end outside saidtank, said bushing comprising: a centrally disposed conductive stud,housing means including a pair of insulating shells surrounding saidconductive stud and insulating said stud from said tank, one of saidinsulating shells being disposed between the outer end of said stud andsaid tank and the other of said insulating shells being disposed betweenthe inner end of said stud and said tank, said housing means formingabout said stud a chamber containing pressurized insulating gas at apressure lower than that of the gas within said tank, stop meansadjustably mounted on the inner end of said stud and bearing rigidlyagainst the inner end of said housing means, an annular sealing platebearing against the outer end of said housing means and closing off theouter end of said chamber, a piston-like member rigidly secured to theouter end of said stud and arranged for telescopic movement within saidsealing plate during contraction and expansion of said stud, saidpiston-like member having a larger external diameter than the diameterof said stud in the region where said stud enters said chamber from saidtank, means providing a seal between said sealing plate and saidpiston-like member, spring means acting between said piston-like memberand said sealing plate for loading said insulating shells in compressionand for holding said sealing plate in selaed relationship to saidhousing means, said spring means being located internally of one of saidinsulating shells, the pressurized gas within said chamber actingthrough said piston-like member to apply tension forces to said studthrough a path etfeotively bypassing said spring means.

2. The apparatus of claim 1 in combination with sealing means at theinner end of said bushing for isolating said chamber from thepressurized gas space within said tank and for precluding pressurizedgas from said tank from leaking around said stud into said chamber, saidsealing means at the inner end of said bushing enclosing a predeterminedarea which is substantially smaller than the area enclosed by thesealing means disposed between said sealing plate and said piston-likemember.

3. In electrical apparatus comprising a tank, a terminal bushingextending through an opening in said tank with its inner end inside saidtank and its outer end outside said tank, said bushing comprising: acentrally disposed conductive stud, housing means including a pair ofinsulating shells surrounding said conductive stud and insulating saidstud from said tank, one of said insulating shells being disposedbetween the outer end of said stud and said tank and the other of saidinsulating shells being disposed between the inner end of said stud andsaid tank, said housing means forming about said stud a chambercontaining pressurized insulating gas, stop means adjustably mounted onthe inner end of said stud and bearing rigidly against the inner end ofsaid housing means, an annular sealing plate bearing against the outerend of said housing means and closing ofi the outer end of said chamber,a piston-like member rigidly secured to the outer end of said stud andarranged for telescopic movement within said sealing plate duringcontraction and expansion of said stud, said piston-like member having alarger external diameter than the diameter of said stud in the regionwhere said stud enters said chamber from said tank, means providing aseal between said sealing plate and said piston-like member, springmeans acting between said piston-like member and said sealing plate forloading said insulating shells in compression and for holding saidsealing plate in sealed relationship to said housing means, said springmeans being located internally of one of said insulating shells, thepressurized gas Within said chamber acting through said piston-likemember to apply tension forces to said stud through a path efiectivelybypassing said spring means.

4. Electrical apparatus comprising a tank containing pressurized fluidat a pressure higher than the pressure external to said tank, said tankhaving a wall containing an opening for receiving a terminal bushing,the terminal bushing extending from outside said tank through saidopening into said pressurized fluid, said bushing comprising a pair ofinsulating shells and a tubular supporting member mounted between saidshells, an annular groove in the outer periphery of said tubularsupporting member, a radially-split locking ring disposed within saidgroove and bearing against an annular inner surface of said tank walladjacent said opening, sealing means disposed between the outerperiphery of said tubular supporting member and the inner periphery ofsaid opening for preventing the leakage of pressurized fluid about saidouter periphery and jack screw means between said tank wall and saidtubular supporting member for bolding said locking ring against saidinner surface.

5. In combination, a tank containing pressurized fluid, a high voltageterminal bushing extending through a wall of said tank, said bushingcomprising a tubular conductive stud and housing means surrounding saidstud, said housing means forming about said stud a chamber isolated fromthe pressurized fluid within said tank and containing pressurizedinsulating fluid at a pressure different from that of the fluid withinsaid tank, an end plate forming an end wall of said chamber and locatedbetween said chamber and the fluid within said tank, said end platehaving an inner peripheral portion closely surrounding the outerperiphery of said tubular conductor, a pair of axially spaced O-ringsdisposed between said inner periphery of the end plate and the outerperiphery of the tubular conductor for respectively sealing the fluidwithin said tank and the fluid within said chamber against leakage alongsaid outer periphery of said tubular conductor, and means formaintaining the space between said O-rings at a lower pressure than thepressure within either said tank or said chamber comprising a passagewaycommunicating at its opposite ends with the space between said O-ringsand the space within said tubular conductor.

References Cited in the file of this patent UNITED STATES PATENTS1,708,859 Acly Apr. 9, 1929 1,870,274 Wulfert et a1. Aug. 9, 19322,117,696 Brandt May 17, 1938 re n..-

